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XR400 & Mikuni Pumper Carburetor question

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Thumper Talk newbie with a question about my bike.

I just bought a 97 XR400 with electric start, stock motor (not built) stock exhaust (but the center tip missing) and a Mikuni Pumper Carb (air box is open from the top and side). I am assuming the carb is unmodified from XR's ONly but the previous owner said he never got the carb/bike running right. I wasn't sure what to make of that but after taking it out for a ride the other day I noted that the bike sputters when I was buzzing down the highway at 60 MPH/2500 feet in elevation. Also, the bike doesn't seem to respond immediately when I blip the throttle to get over a rut in the dirt road. 

Not really sure where to start. I looked at the spark plug and it looks like it's not fat or lean, so I am kinda at a loss of where to go next. opinions would be appreciated.

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Try some obtain booster. It'll REALLY wake that motor up.
Be sure to follow the directions on the bottle, do NOT use too much, you can easily blow the head.
Don't ask how we know.
Pic is of a YZF 250 head, we used too much booster.

YZFHead2_zps5a1db420[1].jpg

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On 9/8/2017 at 7:43 PM, arthur6 said:

Try some obtain booster. It'll REALLY wake that motor up.
Be sure to follow the directions on the bottle, do NOT use too much, you can easily blow the head.
Don't ask how we know.
Pic is of a YZF 250 head, we used too much booster.

YZFHead2_zps5a1db420[1].jpg

Octane does nothing for power. Don't give answers to questions if you haven't got a clue.

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Octane booster?? Now I personally would scrap this idea! Your problem sounds like a jetting issue? The carb may also need a good cleaning? While you have it apart check your jet sizes and see if they match for your low elevation 500' Mikuni has a chart for each size carb and what size jets to use for your elevation and how to adjust the carb. Start there.

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2 hours ago, piltoyougram said:

Octane booster?? Now I personally would scrap this idea! Your problem sounds like a jetting issue? The carb may also need a good cleaning? While you have it apart check your jet sizes and see if they match for your low elevation 500' Mikuni has a chart for each size carb and what size jets to use for your elevation and how to adjust the carb. Start there.

Agree.

A stock engine doesn't need octane booster. Pump premium is plenty.

Take the carb off and clean it, check jet sizes and report back.

We use this .pdf to work on and tune the TM36 Mikuni. http://www.mikuni.com/pdf/hs40_manual.pdf

Do you mean to say that the left side air box door is missing? If you have it, replace it and see if the bike runs any better. You might also block off the top too. If the bike runs better, jetting is too lean for the airbox to be so open as it is currently.

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5 hours ago, piltoyougram said:

Octane booster?? Now I personally would scrap this idea! Your problem sounds like a jetting issue? The carb may also need a good cleaning? While you have it apart check your jet sizes and see if they match for your low elevation 500' Mikuni has a chart for each size carb and what size jets to use for your elevation and how to adjust the carb. Start there.

Thanks for the reply. I was afraid you would say that. I was hoping that I could just make an offering to the Honda gods and all would be well. I'll dig into the carb and get back to all of you.

 

2 hours ago, Trailryder42 said:

Agree.

A stock engine doesn't need octane booster. Pump premium is plenty.

Take the carb off and clean it, check jet sizes and report back.

We use this .pdf to work on and tune the TM36 Mikuni. http://www.mikuni.com/pdf/hs40_manual.pdf

Do you mean to say that the left side air box door is missing? If you have it, replace it and see if the bike runs any better. You might also block off the top too. If the bike runs better, jetting is too lean for the airbox to be so open as it is currently.

Thanks, Trailryder42 for the pdf. Unfortunately, when I bought the bike it didn't come with the side and top covers to the airbox. I was thinking it was lean, but as I told piltoyougram, I'll dig into the carb and report back.

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On 9/28/2017 at 7:54 PM, OrangePeeled said:

Any luck with it?

Brandon

As soon as I made this post I got busy with work. I'm just wrapping up my projects and should be able to dig into it next week. I have since learned that the side covers and top of the airbox are in place. However the exhaust is uncorked (I am missing the interior portion to the muffler), so I wonder if that might be affecting my performance. Regardless, I am going to be putting in a carb kit. so hopefully I'll have an answer next week.

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OK, carb cleaned and new XR's Only exhaust installed. Push button start and she purrs like a kitty. I'm thinking the bike just sat for too long before I purchased it. 

Edited by bgw2go
typo

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odds are the carb clean did the trick. but changing the exhaust so suit the carb will also help balance things out.  

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The previous owner of this bike bought it from a guy who did a full conversion from a EX400, wiring harness, starte,r crank, Etc. I like it but it has its quirks. It doesn't always kick the bike over and there's the added weight. The upside though is when you stall at a traffic light...you don't have to kick start it & that's worth it to me.

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